The overall objective of this project is to obtain a detailed understanding of the molecular structure of the action and myosin filaments of relaxed vertebrate smooth muscle, and their arrangement in the contractile apparatus. Using these structures as a starting point, we also wish to determine how changes in the arrangement of the molecules or filaments are related to contraction of smooth muscle. The results should provide fundamental insights into the molecular mechanism of smooth muscle contraction and its regulation, and provide a basis for understanding structural defects that may occur in diseased states.